1. Field of the Invention
The present invention relates to a stick type ignition coil attached inside a plug hole provided on each of the engine cylinders of an internal combustion engine.
2. Related Art
One type of known stick type ignition coil has a coil case, primary and secondary coils and magnetic cores. The coil case is made of resin and has a cylindrical shape. The primary and secondary coils and the cores are accommodated in the coil case. The coil case has a connector housing at the upper part of the coil case to connect the primary coil to a battery. The coil case also has a high-voltage tower portion at the lower part of the coil case. The high-voltage tower portion is to be inserted into a plug hole provided on a cylinder head of an engine cylinder, and is attached to a spark plug so that a high-voltage terminal (a secondary terminal) of the secondary coil is electrically connected to the spark plug.
In the above-described stick type ignition coil, an outer diameter of the ignition coil is limited by an inner diameter of the plug hole; that is, the outer diameter of the ignition coil must be smaller than the inner diameter of the plug hole. Therefore, to improve boosting (voltage amplification) performance of the ignition coil, a longitudinal length of the coil case is increased so that a number of windings of the primary and secondary coils can be increased. Further, to improve voltage proof performance inside the coil case, the coil case is formed thinner to increase insulating space.
According to such ignition coil, insulating resin such as polybutylene terephthalate (PBT), polyethylene terephthalate (PET) or polyphenylene sulfide (PPS) are used for forming the coil case. JP-A-8-339928 discloses that PPS is suitable for forming the coil case due to its excellent flowability.
However, when the coil case is formed thinner or elongated further to improve performance of the ignition coil, an integral molding to form the coil case becomes difficult even when the coil case is made of PPS which is preferable to mold. Further, if the coil case has an undercut portion, the structure of a molding die for the integral molding becomes extremely complicated, resulting in a high molding cost.